Recently, reflective films are used in the fields of a reflective plate for use in a liquid crystal display, a member for a projection screen or a planar light source, a reflective plate for a lighting apparatus, and an illuminated sign. In the case of, for example, reflective plates in liquid crystal displays, reflective films having high light reflectance are demanded in order to supply light to liquid crystals as much as possible to improve the performance of the back light unit in response to the requirements of providing a larger screen of a liquid crystal device and higher displaying performance.
Further, as a display device in, for example, a notebook-type computer, there has been used a liquid crystal display device provided with a back light unit and liquid crystal display elements, which can be designed to have a reduced thickness and provide images that are easy to watch. Such back light units mostly include back light units of an edge-light system that has a transmission light guide plate provided on one end thereof a linear light source such as a fluorescent tube. In most of such edge-light systems, one side of the light guide plate is partially covered with a light diffusion material and that side is entirely covered with a reflective material to constitute a planar light source. Such a reflective material is required to have a high reflecting performance.
Japanese Patent Application Laid-open No. Hei 4-239540 and Japanese Patent Application Laid-open No. 2002-138150 disclose white polyester films obtained by drawing a sheet comprised by an aromatic polyester-based resin and a filler to form minute voids in the sheet to allow light scattering reflection. However, these films fail to provide the required high light reflectance. Further, the aromatic rings contained in the molecular chains of the aromatic polyester-based resin that constitutes the films absorb ultraviolet rays, so that the films are deteriorated and yellowed due to the ultraviolet rays from the light source of the liquid crystal display device and the like. This leads to the disadvantage of a reduced light reflectance of the reflective film.
Further, Japanese Patent Application Laid-open No. Hei 11-174213 discloses a porous sheet obtained by drawing a sheet made from a polypropylene resin to which an inorganic filler is added. In this case, it is necessary to add the inorganic filler in an amount of 60% by weight or more, so that there arises the problem that the film forming ability is poor.
Further, Japanese Patent Application Laid-open No. Hei 6-298957 discloses a film obtained by mixing a resin having a specified refractive index and an inorganic filler and drawing a film made from the mixture. Japanese Patent Applications Laid-open Nos. Hei 7-230004, Hei 7-287110, and Hei 8-262208 disclose reflective sheets obtained by mixing a polyolefin-based resin and an inorganic filler and drawing a film made from the mixture. Japanese Patent Application Laid-open No. Hei 11-149816 discloses a reflective film obtained by mixing a polyolefin and barium sulfate and drawing a film made from the mixture. In these case, however, a measure for improving the drawability, for example, addition of a processing auxiliary must be taken. Moreover, although the resultant film has an increased light reflectance by drawing, there arises another problem that it suffers from not a small size change with time or in a heated environment due to the phenomenon of orientational relaxation.